The gap between the incomes of the rich and the poor seems to have widened over the past thirty years. What does it mean? Does it mean some people’s incomes have always been high, other people’s will always be low, and the gap has widened in real terms? Or does it mean people who are young and poor now have better opportunities than young people had thirty years ago – people are earning more as they age than their counterparts earned thirty years ago, thus producing a gap between poor twenty-year-olds and rich fifty-year-olds that is larger now than it was thirty years ago?